1. Introduction
The provision of a supply of air at positive pressure to the entrance of a patient's airways for treatment of SDB was first disclosed in U.S. Pat. No. 4,944,310 to Sullivan. The delivery of the pressurized flow is facilitated by a patient interface, such as a mask. Mask systems may be classified as “nasal masks,” “full-face masks,” “nose and mouth masks” and a variety of nozzle designs, including “nasal pillows,” “nasal puffs,” “nasal prongs” and “nasal cannulae” designs. An example of a nasal pillow-type mask system is disclosed in WO 2004/073778 (Gunaratnam et al.), the contents of which are hereby incorporated by reference.
While different manufacturers use different terminology to refer to different components, patient interfaces systems, also called mask systems, typically comprise:
(i) a cushioning element
(ii) a headgear system to position and retain the cushioning element in position;
(iii) a rigid structure, known as a frame or shell; and
(iv) an air delivery tube.
1.1 Cushioning Element
The cushioning element, or cushion, generally includes a soft, conforming structure made from a material such as a silicone, a gel, or a foam. In use the cushion is held against the appropriate part of the face to effect a seal. The cushioning element should form an adequate seal with the entrance to the airways in order to maintain sufficient air pressure for splinting open the airways. In some cases it may not be necessary to form a complete seal provided an adequate supply of air can be provided at appropriate pressures and flow rates for effective therapy.
Nasal pillows and nasal puffs form a seal on the outside of the nares, whereas nasal prongs and nasal cannulae are positioned further into the nares and may form a seal on an inside surface of a nare, rather than an outside surface. The location of the seal is a consideration because different surfaces have different orientations, meaning that the force vector needed to form a seal may have a different direction. This may result in a headgear that is appropriate for one design being inappropriate for another. Furthermore, different seal types may be preferred by different patients, and may be regarded as being more comfortable by some patients.
1.2 Headgear System
A mask system typically further comprises a range of frame and headgear systems intended to provide force vectors of appropriate magnitude and direction to hold the cushion in place.
1.3 Frame
Many mask systems include a rigid, or semi-rigid, structure referred to as a shell or frame. Together the cushioning element and the frame may define a chamber. Typically, the cushion, headgear and an air delivery tube are attached to the frame. The frame serves as an anchoring point for the cushion, headgear and tube. Past efforts in mask design have been directed towards mechanisms for anchoring the frame in a fixed position with respect to the face and then attaching the cushion to the frame to form a seal.
1.4 Tube
Many mask systems require some form of tube, hose, or conduit to deliver the flow of air to the mask for breathing by the patient. In use, tube drag forces can disrupt the effectiveness of the seal. This can be the result of the weight of the tube and/or movement of the patient. While tube drag can be alleviated to some extent by the use of swivels, ball and socket joints, and tube anchoring arrangements, many patients feel the need to over-tighten headgear straps in an attempt to reduce the problem, leading to discomfort.
2 Prior Art
Design of an effective respiratory mask system requires consideration of many factors. Apparently subtle changes my improve or decrease comfort or effectiveness.
While millions of people suffer from the condition of sleep disordered breathing, many fail to comply with therapy because of problems with comfort, ease of use, stability, leak, and obtrusiveness, and thus expose themselves to health risks.
Some prior art mask systems attempt to anchor a rigid frame in a fixed position with respect to the face and require patients to increase headgear tension to a level sufficiently high to overcome the seal disruptive effects of patient movement, and/or tube drag.
Whilst some smaller patient interfaces (such as some nasal puffs, and some nasal prongs) may be less obtrusive than larger masks (such as a full-face mask) they can suffer from a lack of stability. Interface stability of such smaller interfaces was improved by holding the frame in a fixed position in front and below the patient's nares. The cushion, including the nozzles, extends from this frame to the nares. The frame was held in a rigid fashion (i.e. a set location) aiming to ensure correct alignment of the pillows.
Some prior art patient interfaces included swivel elbows or ball-and-socket joints.
Some headgear designs incorporated semi-rigid elements that increased stability of the mask frame. Some frames were designed having a horizontally central tube attachment point.
Some prior patient interfaces have required patients to increase headgear tension forces to a high level in an attempt to overcome disruptive forces. This can lead to excessive forces on sensitive regions of the face, resulting in discomfort to the patient.
Referring to FIG. 1, a prior art respiratory mask system may include a cushion 24 comprising, for example, nasal pillows. The cushion 24 is supported on a rigid frame or shell 18 and a flexible component 22, for example a gusset, is provided between the cushion 24 and the frame 18. An air delivery hose or tube 16 is connected to the frame 18 for the delivery of the flow of pressurized breathable gas. A rigid headgear 20 is connected to the frame 18 to maintain the cushion 24 in sealing contact with the nose of the patient 1.
Cushions which may be used in such a prior art respiratory mask system as shown in FIG. 1 include those disclosed, for example, in ResMed Ltd.'s Swift® LT.
FIG. 2 schematically illustrates another respiratory mask system according to the prior art, the Fisher & Paykel Infinity® 481 mask. A cushion 24 comprising nasal prongs 23 is attached to a mask frame or shell 18. The mask frame includes headgear connectors 26 provided on sides of the mask frame 18. A vent including a plurality of vent holes 28 is also provided in the mask frame 18.
The mask frame 18 is connected to an air delivery hose or tube 16 by a swivel elbow 17. The air delivery tube 16 is connected to a flow of pressurized breathable gas, such as generated by a blower or flow generator, by a coupling element 30.
As the headgear is connected to the mask frame 18 by the headgear connectors 26, any relative movement between the mask frame 18 and the patient's face may result in a disruption of any seal that may have formed.